This invention relates to an audible low-fuel alarm for a propane fuel tank such as is used with hot air balloons to project heat into the balloon envelope. The alarm according to the invention described in this application is particularly adapted to be used in connection with a particular type of prior art fuel indicator. The fuel indicator comprises a fuel gauge which is mounted in the top of the propane tank. The fuel gauge includes an indicator needle which is controlled by a rotatably mounted magnet. The fuel gauge itself does not directly communicate with the interior of the propane tank. Rather, a probe is screwed tightly into an access port in the propane tank directly beneath the gauge. A mechanical float senses the amount of propane in the tank and rotates a magnet rotatably mounted on top of the probe in spaced-apart relation to the magnet in the fuel gauge. As the level of propane in the tank drops, the magnet positioned on top of the probe rotates and by magnetic attraction through the wall of the propane tank, rotates the magnet connected to the indicator needle of the gauge.
There are known types of audible alarms which can be used to indicate the existence of low-fuel level or any other fluid in a reservoir. However, these devices typically require insertion within their own port communicating with the interior of the reservoir. This is a particularly undesirable and impractical procedure when dealing with propane tanks. Propane tanks are filled under high pressure, making it essential to have the least possible number of ports in the tank walls, since each port weakens the tank structure and provides a possible location for leakage.
Futhermore, the prior art fuel indicator described above is of necessity located directly on top of the propane tank and very close to the top of the tank because of the requirement that the magnet within the gauge and the magnet on top of the probe be close enough together to be within each other's field of magnetic attraction. This creates a particular problem in hot air balloons, since the fuel tanks are typically stored in the corners of the balloon carriage and tucked under the carriage overhang in order to conserve space for the occupants. For this reason, the gauge cannot easily be seen. The audible alarm therefore serves the purpose of alerting the balloon pilot that the fuel within a particular propane tank has reached a predetermined low level.
Another factor involved in the choice of an audible alarm for a propane fuel tank is the requirement that all such devices be approved by the Federal Aviation Administration. It is therefore particularly desirable to provide an audible alarm which can be used in combination with prior art visual indicators which are already FAA approved, and to have a built-in redundancy in case of malfunction.